1. Field of the Invention
The present invention relates to a tabular silver halide emulsion which is useful in the field of photography, and also to a method of preparing the same.
2. Description of the Related Art
Tabular silver halide grains having parallel twin planes (hereinafter referred to as "tabular grains") have the following photographic properties:
1) Since they have a large ratio of the surface area to the volume (hereinafter called "specific surface area"), a great amount of a sensitizing dye can be adsorbed to their surface. As a result, they have color sensitized-sensitivity which is high relative to their intrinsic sensitivity.
2) When an emulsion containing the tabular grains is coated and dried, the planes of the grains are arranged in parallel on the surface of a support, whereby the coated layer can be so thin that sharpness is good.
3) In the case of same sensitivity, the amount of silver coated can be smaller than in the case of using a non-tabular grain emulsion, and the sensitivity/graininess ratio is therefore high.
4) They are highly resistant to natural radiation.
Having these many advantages, the tabular grains have hitherto been used in commercially available materials.
An ideal feature which tabular grains must have to demonstrate their advantages fully is high aspect ratio. Tabular grains having a high aspect ratio, however, will have the following defects if prepared by the known method.
1) They have a broad distribution of diameter of projected area.
2) The tabular grains exist along with bar-shaped ones, tetrapod-shaped ones, ones having single twinning, or ones having non-parallel twin planes.
3) Hexagonal tabular AgX grains (hereinafter called "hexagonal tabular grains") and triangular tabular AgX grains (hereinafter called "triangular tabular grains") exist together as the tabular grains.
Therefore, the tabular grains are disadvantageous in the following respects:
1) They cannot be expected to achieve hard gradation (i.e., so-called high gamma) in a characteristic curve.
2) When they consist of large ones and small ones, optimal chemical sensitization cannot be performed on them since the best conditions for chemical sensitization of the large ones differ from those for chemical sensitization of the small ones.
3) Since hexagonal tabular grains and triangular tabular grains exist together, they have non-uniform chemical-sensitization characteristic since these types of grains differ in chemical sensitization readiness.
4) The portion of the largest diameter of a hexagonal tabular grain has 1.23 times greater than that of the largest diameter of a triangular tabular gain having the same projected area. Hence, triangular tabular gains, if existing along with hexagonal ones, will deteriorate graininess.
5) A multi-layered structure which has an upper layer formed of monodisperse large grains and a lower layer formed of monodisperse small grains utilizes light more efficiently and hence has higher sensitivity than a single layered structure which has an emulsion coating layer which contains large grains and small grains together. This advantage cannot be utilized well.
Generally, tabular gains are formed in three steps, i.e., nucleation, ripening, and grain growth. Since it is impossible to form only tabular grains in the nucleation step, the grains other than the tabular ones must be eliminated in the ripening step. The monodispersibility of the tabular grains is determined during the nucleation and ripening steps. In the method of preparing tabular grains having a low aspect ratio of 8 or less, solvents for silver halide, representative examples of which are ammonia and thioether, are used to eliminate the grains other than tabular ones and to enhance the monodispersibility of tabular grain nuclei as is disclosed by Saitoh in JP-A-2-838, JP-A-1-1311541, JP-A-2-28638, and JP-A-63-11928. ("JP-A" means Published Unexamined Japanese Patent Application.) A method is disclosed in which ammonia ripening is performed, thereby forming tabular grains which have an average aspect ratio of 5.8 and which have a variation coefficient of 10.5% and thus excel in monodispersibility.
However, it is not desirable that a solvent such as ammonia or thioether be used to dissolve silver halide in the step of forming grains which have a high aspect ratio. This is because, although the solvent eliminates the grains other than tabular ones to enhance the dispersibility among the tabular nuclei, it deforms the tabular nuclei into spherical one or the like. Consequently, each tabular grain becomes thicker, failing to have a high aspect ratio. It would therefore be very difficult to form tabular grains having a high aspect ratio, without using solvents for silver halide, due to the fact that the tabular grains mix with other grains and are broadly distributed in terms of size. No solvents may be used for silver halide, and only physical ripening (known as Ostwald ripening) is performed sufficiently long, thereby to eliminate the grains other than the tabular ones. In this case, however, the ripening of the tabular grain each other is promoted such that small tabular grains become smaller and the large grains become larger, inevitably increasing the size distribution among the tabular grains.
With regard to preparation of tabular grains having a high aspect ratio, Zola et al. disclose in JP-A2-222940 a method of preparing silver iodobromide grains which exhibit monodispersibility and which are characterized in that the quotient obtained by dividing the aspect ratio by the variation coefficient is over 0.7. Also, Brust discloses in International Disclosure 92/07295 a method of preparing silver iodobromide grains which exhibit monodispersibility and which are characterized in that the quotient obtained by dividing the aspect ratio by the variation coefficient is over 1.2. Either embodiment, however, does not provide tabular grains which have a variation coefficient of less than 20% and an aspect ratio ranging from 8 to 30 and which are therefore considered useful in monodisperse tabular grain emulsions practically used as photographic emulsions. U.S. Pat. Nos. 5,147,771, 5,147,772 and 5,147,773, all to Tsaur et al., which were recently published, disclose methods of decreasing the size distribution of grains by the use of polymers during the forming of grains. These patents describe method of manufacturing emulsions much excelling in monodispersibility, containing grains which have an aspect ratio of 8 or more and a variation coefficient of less than 10%. However, it is pure silver bromide emulsions which exhibit excellent monodispersibility; none of the silver iodobromide emulsions excelling in photographic properties have an aspect ratio of 8 or more and a variation coefficient of less than 20%. The higher the content of iodide ions, the more difficult it would be to achieve high monodispersibility. In other words, the projected area distribution of the grains greatly broadens as iodide ions increase in number. If the grains are made to contain a high-silver iodide region, the aspect ratio will decrease remarkably as is known in the art. To place a high-silver iodide region within a grain is very desirable for the purpose of improving sensitivity, graininess, and resistance to pressure. The practical importance of silver iodobromide emulsions containing silver iodide and silver iodobromide emulsions having a high-silver iodide region in the grains are well known. Nonetheless, it has been desired that techniques be developed to prepare such an emulsion which has not only a high aspect ratio but also a narrow grain-size distribution.